This invention relates to an electro-acoustic transducer and, more particularly, to such a transducer that is provided with a novel vibratory structure.
In a typical electro-acoustic transducer, a vibratory structure is coupled to an electromagnetic coil disposed in magnetic flux such that, if current is supplied to the coil so as to impart motion thereto, the vibratory structure likewise moves to generate sound waves. This, of course, is the basic construction of a loudspeaker, headphones or other electrical-to-sound converter. Alternatively, if the vibratory structure is subjected to a mechanical force, such as by sound waves impingent thereon, the movement of the vibratory structure is transferred to movement of the electromagnetic coil, whereupon currents are induced in the coil which is moving in the presence of magnetic flux. This type of transducer is typical of a microphone.
Generally, in electro-acoustic transducers of the aforementioned type, the vibratory structure is a membrane, a diaphragm, a bobbin, or the like, and should be constructed of materials having a high Young's modulus (E) and a low resonant sharpness, or quality factor (Q) for desirable acoustic characteristics. Furthermore, such vibratory structures should be light weight. If the vibratory structure is formed of material having a high Young's modulus, the structure is more rigid and, therefore, when it is driven, results in a high fidelity conversion of electrical energy to sound or mechanical energy to an electrical signal. Furthermore, if this material exhibits a relatively low quality factor Q, peaks or valleys in its frequency characteristic, which are associated with higher quality factors, are avoided. That is, with a high quality factor Q, the vibratory structure undergoes abnormal vibrations when the driving force supplied thereto coincides with, or is in the region of, its resonant frequency. Such abnormal vibrations result in the aforementioned peaks or valleys in the frequency characteristic. With a low quality factor, such abnormal vibrations are avoided. The vibratory structure preferably should be light weight so as to minimize its inertia. That is, a relatively heavy vibratory structure used in, for example, a loudspeaker, cannot follow accurately an input driving force, particularly at middle and higher frequencies. This means that, in such a loudspeaker, the sound reproducing characteristics thereof are degraded, particularly in the medium and high range regions. A similar disadvantage also is applicable to a relatively heavy vibratory structure used in a microphone.
Another difficulty encountered with electro-acoustic transducers relates to the high operating temperatures of the voice coil, that is, the current conductor which is disposed in the magnetic flux, when the transducer is driven. More particularly, the heat which is developed at and in the vicinity of the voice coil is sufficiently high that the vibratory structure (i.e., the diaphragm, membrane or bobbin) should be formed of materials that can withstand high temperatures. For example, if the voice coil is wound upon a bobbin, the bobbin may be deformed at the high operating temperature of the transducer, thereby creating the possibility that the voice coil may slip, which could damage or destroy the transducer. Even if the voice coil does not slip from the bobbin, deformation of the latter may result in undesired contact between the bobbin and the magnetic circuit of the transducer, which also could damage the transducer. Still further, by reason of the deformation of the bobbin, the voice coil may not uniformly traverse the magnetic flux in the magnetic circuit. This non-uniformity may result in undesired distortion of the sound (or sound signal) which is produced. Thus, it is important that the vibratory structure be formed of sufficiently heat-resistant material.
One type of material that is light weight and heat resistant is the polyimide film. If a polyimide film is used in the construction of the vibratory structure, such as the membrane, diaphragm or bobbin of an electro-acoustic transducer, the aforenoted desirable properties may be attained, while avoiding undesired temperature-dependent characteristics. However, heretofore, the voice coil has been mechanically coupled to the polyimide film by an adhesive. For example, in a so-called ribbon-type speaker in which the voice coil is coupled to the diaphragm, or in a speaker in which the voice coil is wound upon the bobbin, if the diaphragm or bobbin is formed of material which includes the polyimide film, the voice coil is adhesively coupled to that film. The high operating temperatures which are developed during operation of the transducer weakens or even breaks the adhesive coupling such that the voice coil may separate from the diaphragm or bobbin. This is attributed, primarily, to the generally poor temperature characteristics of typical adhesives which are needed to couple the voice coil to the vibratory structure.